1. Field of the Invention
The present invention relates to a device for communication between one or more external buses and a set of electronic modules installed in a local unit such as an electronic bay.
It can be applied especially but not exclusively to electronic devices or machines installed on board aircraft, these machines being interconnected by one or more buses known as external buses. Certain of these machines, especially computers, are integrated into racks, each rack assembling a set of modules interconnected by an internal bus called a &lt;&lt;backplane bus&gt; &gt;
2. Description of the Prior Art
Given that the number of interconnected electronic machines installed on board aircraft is on the increase, it is necessary to have available an interconnection means to reduce the number of physical links between machines while providing for an increasingly higher bit rate. At present, the trend is towards an approach that uses a multi-transmitter and multi-receiver, multiplexed bus as defined by the aeronautical standard ARINC 629.
The fact is that this standard implies complicated mechanisms of detection and tolerance to malfunctioning, given the critical nature of the information exchanged, for these mechanisms are made up of specific components that are distributed in small quantities and are therefore costly. This standard also implies insufficient bit rates (of 2 Mbits/s) when the operations to be carried out are maintenance operations such as, for example, remote software-loading operations.
There is also the known IEEE standard 802.3 implemented for example in Ethernet type local area networks designed for communications at higher bit rates (10 Mbits/s) between computers, not specifically for the field of aeronautics but for applications in data processing in general. The components used for these communications are therefore designed for increasingly wider distribution, have a higher level of integration and are more simple because communications of this type implement a simple protocol not subjected to constraints that are as severe in terms of detection and tolerance to malfunctioning
It is therefore advantageous to apply this standard to communications between functional units connected to the external bus.
However, this standard has been designed for communications over distances ranging from some meters to some hundreds of meters and not for communications between modules at distances of some centimeters from one another in an electronic bay or rack.
Furthermore, this standard applies the CSMANCD principle (or principle of carrier sense mode access/collision detection) which consists in monitoring before transmitting, and transmitting only when the channel is free, the state of the channel being determined by the detection of the carrier. In the event of collision, i.e. when several transmitters start transmitting at the same time, each transmitter waits for a period with a random duration before making a new attempt at transmission.
Thus, from a certain frequency of transmission requests onwards, the number of collisions increases, leading to a collapse of the mean bit rate of the link.
It is also a fact that the requirements in terms of transmission bit rate between modules of one and the same rack are very great and may exceed the instantaneous capacity of a single link of the ARINC 629 or Ethernet type. The ARINC 659 standard has therefore been devised. This standard defines a redundant bus with a high bit rate (60 Mbits/s) that is also a multiplexed multi-transmitter and multi-receiver bus.
However, transmissions at such bit rates are very sensitive to electromagnetic disturbances and therefore require the use of costly technologies.
Furthermore, if different protocols are used on the external bus and the internal bus of a rack, the data formats and the respective bit rates of these buses are different. These buses are therefore asynchronous with respect to each other. The coupling modules providing for the interconnection between the external bus and the modules of the rack then inevitably introduce a delay time in the transmission of information elements, and must perform an operation for the manipulation of information elements that reduces the reliability of the entire communication system.